The present invention generally relates to a dry cell or dry battery, and more particularly, to a cylindrical type Leclanche cell (referred to merely as a dry cell hereinafter) which is free from the danger of rupturing due to an increase of an internal pressure thereof through improvements of its bottom plate for higher safety.
It has been a recent trend that the dry cell has been changed from a paper tube cladding or paper tube jacket type to a metal cladding or metal jacket type, with a marked development on its cell performance through improvements on resistance against leakage and also resistance against expansion or swelling thereof. For the metal jacket type as referred to above, there have generally been employed a double-lock type jacket folded into double layers at its opposite edges, a butted-joint type jacket merely butted to each other at opposite edges thereof, and a laser-welded type jacket subjected to laser welding at its opposite butted edges (referred to as a laser-welded type jacket hereinafter), etc.
Subsequently, construction of an outstanding dry cell B conventionally employed and adopting the metal jacket as referred to above will be described with reference to FIGS. 1 to 3.
As shown in FIG. 1, the dry cell B generally includes a zinc can z containing therein a cathode mixture m, a carbon rod c, a partitioning paper p having a paste layer over its one face, etc., a synthetic resin sealing member 2 for sealing an opening 1 of the zinc can z, a heat-shrinkable synthetic resin tube 6 covering the entire outer periphery of the zinc can z including a bottom plate 4 (anode) at the bottom portion 3 of the zinc can z, a seal packing 5 and a shoulder portion of the sealing member 2, a sealing material 7 such as pitch or the like disposed on a recess formed on the upper surface of the sealing member 2, and a sealing plate 8a integrally formed with a cap 8 (cathode) and disposed on said sealing member 2. The entire dry cell is further clamped, through a resinous packing 9 disposed on the sealing plate 8 and the seal packing 5 provided at the lower portion of the dry cell, by a metallic outer cladding can or metal jacket 10 (referred to as a metal jacket hereinafter) to maintain the dry cell in an air-tight state.
When the laser-welded type metal jacket as shown in FIG. 2(B) is employed as a metal jacket instead of the double-lock type metal jacket (FIG. 2(A)) or butted-joint type metal jacket (FIG. 3(B)), as compared, for example, with the double-lock type metal jacket in FIG. 2(A), since the laser-welded portion 22 has thickness smaller than that of the double-lock portion 21 by a difference a, the laser-welded type metal jacket of FIG. 2(B) can employ a raw cell 23' having a larger external diameter than the raw cell 23 of the double-lock type metal jacket of FIG. 2(A), with a consequent increase of the cell capacity by that extent. Moreover, since the laser-welded metal jacket (FIG. 3(A)) has a tensile strength higher than that of the butted-joint type metal jacket (FIG. 3(B)) in a diametrical direction of the jacket especially at the laser-welded portion 22, said laser-welded metal jacket is free from any separation or opening w at a butted-joint portion 31 as in the butted-joint type metal joint (FIG. 3(B)) even during over-discharging.
In the case where a dry cell employing the laser-welded type metal jacket (FIG. 2(B)) is wrongly used, for example, by undesirable charging thereof or by leaving it in a high temperature atmosphere above 85.degree. C., etc., the dry cell may be expanded due to an increase of its internal pressure through generation of a gas therein. The gas thus generated passes through a gap between the zinc can z and the resinous sealing member 2 so as to be collected between the zinc can z and the synthetic resin tube 6, while part of the gas is discharged outside via the seal packing 5 provided at the bottom portion of the zinc can z (FIG. 1). If the internal pressure within the dry cell is rapidly raised, since the tensile strength in the diametrical direction is extremely high in the case of the laser-welded type metal jacket (FIG. 2(B), FIG. 3(A)) as compared with that of the double-lock type metal jacket (FIG. 2(A)) or butted-joint type metal joint (FIG. 3(B)), the dry cell is expanded longitudinally in the direction of its height, thus resulting in such troubles as disengagement of the cap 8 integrally formed with the sealing plate 8a and provided at the upper portion or scattering of the cathode mixture m. Particularly, in the case of scattering of the cathode mixture m, appliances employing the dry cell may be heavily damaged, giving rise to various accidents in some cases.